Five charged over killing of Russian opposition leader Boris Nemtsov

All five suspects being held over the brazen murder of Russian opposition leader Boris Nemtsov have been charged with taking part in a contract killing.

Moscow city court said in a statement that the accused - all from Russia's volatile North Caucasus region - had been charged with murder carried out for financial gain.

Former deputy prime minister Mr Nemtsov was shot dead in front of the Kremlin in central Moscow on February 27 in the highest profile slaying of a political critic in the fifteen years that President Vladimir Putin has been in charge.

Five men, including prime suspect Zaur Dadayev - a former high ranking Chechen police officer - were detained a week after the murder.

Dadayev at first confessed to the killing but later retracted his statement, with rights activists saying he was likely tortured into admitting his guilt.

Close allies of Mr Nemtsov have dismissed official hypotheses that the opposition politician's death could be linked to his support for the publication of cartoons of the prophet Mohammed in French weekly Charlie Hebdo, given that all the suspects are Muslim.

Protesters march against the death of Boris Nemtsov. (AAP) (AAP)

Russia's opposition has accused the Kremlin of being behind the assassination, which occurred in a heavily policed area as Mr Nemtsov was working on a report about the involvement of Russian troops in the Ukraine conflict.

The Kommersant daily has quoted an anonymous source as saying that investigators had identified the person who supplied the gun and car to the suspects and promised to pay them around 80,000 euros each for the hit.